Two LANs Aren't Better Than One

By SmallBusinessComputing Staff | Posted August 18, 2004

Currently, I have two local area networks (LANs) set up in my house. In my living room, I have a cable modem that connects to a Linksys WRT54GS wireless router. The only system on this LAN is a notebook that connects to the router.

In my office, I have a second LAN made up of a Linksys BEFSR41 wired router and a Linksys WET54G wireless bridge (192.168.1.103). I've connected one desktop PC and one laptop computer to the BEFSR41 router via Ethernet cables. The office LAN currently has no Internet access.

I'd like to be able to access a cable modem Internet connection in my living room when using the desktop and laptop in my office, which is why I installed the Linksys WET54G wireless bridge on the office LAN. Currently, I can access all the configuration utilities on the various routers and ping each of the PCs, but, for some reason, I'm unable to browse the Internet. Since I can ping all of the devices, I know both networks are bridging successfully. I just can't seem to gain access to the Internet. At this point I don't know what else to try. Any ideas?

Thanks for any information you can send my way!

If I understood your question correctly, you have two LANs in your home. The living room LAN is connected to the Internet via a cable modem. The office LAN has no Internet connection, but is connected to a router nonetheless. To give the office PCs Internet access, you decided that it would be best to add a wireless bridge.

With that being said, I have a few comments. For starters, why are you using a router on the office network? If there is no Internet connection available for that network, why use an expensive and complicated router when a simple hub or switch would do?

Additionally, because of the presence of the wireless bridge, you're basically using two routers on a network that clearly needs only one. This isn't to say that you couldn't get the network to function with the second router present, but it is going to add a layer of complexity to the project that you can easily avoid.

Lastly, DHCP requests cannot be granted to PCs across a router. (DHCP, short for Dynamic Host Configuration Protocol, assigns dynamic IP addresses to devices on a network. With dynamic addressing, a device can have a different IP address every time it connects to the network).

It's very possible that you're office LAN and your living room LAN each has its own DHCP server. If that is the case, it would explain why you can't get the office PCs to access the living room LAN's Internet connection. The default configuration on the office router routes unknown packets to its own Internet gateway. Since it doesn't have an Internet connection of its own, those requests fail.

Even though the bridge is configured with the correct Internet gateway, it's designed to be used with a single machine. When you connect a bridge directly to a PC, the computer's NIC card acquires the bridge's network settings. That's why when you run an IPCONFIG of the system with the bridge attached to it you see the IP address that was originally assigned to the bridge.

When you connected it to the switch, those settings don't get passed on to the other systems on the network. For that to happen it would need to perform a Network Address Translation (NAT) type function. (NAT is an Internet standard that lets a LAN use one set of IP addresses for internal traffic and a second set of addresses for external traffic.)

Now that we understand the problems, let's talk about the possible solutions. The thing that confuses me the most about this configuration is the fact that since the wireless signal between the living room and office LANs is strong enough to support the wireless bridge, why wouldn't you have just installed wireless network adapters in the PCs themselves?

I would pull the router from the office LAN (especially since right now it serves no function), take the wireless bridge and connect it directly to the desktop PC. Then I would purchase and install a wireless NIC in the notebook. You can use either a PC Card or a USB wireless adapter.

The D-Link DWL-650+ is a good, inexpensive PC Card, and the D-Link DWL-122 USB Wireless Adapter would work as well. You can buy one for about $30. This would give both of your office PCs access to the living room LAN without all of the headaches associated with trying to configure the network to function with the second router.

If you don't want to do that, you can replace the router in the office with a switch (also about $30). Connect the wireless bridge and the two PCs to the switch. Assign each PC a static IP address based on the living room network segment (192.168.1.x). Give each PC the gateway address of the living room router (192.168.1.5). I would reconfigure this address as 192.168.1.1. You don't have to, but it's a good idea.

By configuring the network this way, the bridge can supply a connection to the living room network. The static IP address fixes the problem of DHCP not passing over the router and the bridges lack of NAT capabilities. And by pointing each system to the correct Internet gateway, it guarantees that unknown packets will be forwarded to their appropriate destination and not simply discarded.

If I were in your shoes, I'd go with the first solution. However, either method should work fine. I hope this helps you. Good Luck!

Adapted from PracticallyNetworked.com, part of the EarthWeb.com Network.

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